1. Field of the Invention
The present invention relates to pressure relief valves which open when a pressure differential across valve exceeds predefined level.
2. Description of the Related Art
Pressure relief valves are provided at various locations in a hydraulic system to limit the maximum pressure to less then a predefined threshold level. For example, a pressure relief valve is normally provided at the outlet of a pump so that the supply line cannot be pressurized to an excessively high level. Should the pressure threshold level be exceeded, the relief valve opens creating a path usually to the hydraulic system reservoir, or tank. Another pressure relief valve is frequently provided at the workport of a control valve assembly to which a hydraulic actuator, such as a piston and cylinder combination, is connected. This arrangement protects against shock loading or inertia overloading due to the load which acts on the piston and cylinder. If that load creates an excessively high pressure within the hydraulic lines attached to the cylinder, the workport pressure relief valve opens to relieve the excessive pressure to the system tank. The workport relief valve remains open until the load condition no longer exists and pressure within the associated hydraulic line decreases below the threshold of the relief valve.
A common type of relief valve has a movable element biased against a valve seat by a spring to maintain the valve in the closed state. When pressure acting on the valve element exceeds the force of the spring, the valve element moves away from the seat, thereby opening the pressure relief valve. In this type of valve, the valve opens as soon as the spring force is exceeded and closes immediately upon the pressure dropping below the spring force.
In some relief valve applications, it is desired that the valve not open immediately when the pressure threshold is exceeded, but rather require that the excessive pressure should be required to exist for a given amount of time. Thus, the pressure relief valve will be immune from sporadic, short duration pressure excursions which exceed the threshold level. Nevertheless, when the pressure exceeds the threshold setting for longer than the given amount of time, the valve opens to relieve that pressure before damage to the hydraulic system can occur. Inversely, it is also desirable in certain situations to delay the valve closing to ensure that the pressure remains below the threshold. This prevents a momentary pressure decrease from closing the valve in a situation where pressure relief is still required.
Therefore, it is desirable to provide a pressure relief valve with bidirectional damping of the valve action, so as to provide hysteresis.